1. Field of the Invention
The present invention relates to a UV-radiation-absorbing, especially solarization-stable, borosilicate glass for a gas discharge lamp, which is characterized by only a slight absorption in the visible range. It also relates to a process for making from a refined glass melt and to a glass discharge lamp made with this UV-radiation-absorbing borosilicate glass.
2. Related Art
Glasses for making gas discharge tubes, like those used in fluorescent lights, are known. These glasses should have a great resistance to solarization as well as great UV absorption properties. Also they should be resistant to the aggressive or reactive conditions existing in these types of glass discharge tubes.
Fluorescent lights are used as light sources, especially for making liquid crystal display devices (LCD) and display devices illuminated from the rear, the so-called backlight displays. Fluorescent lights for these applications have very small dimensions and correspondingly the lamp glass only has an extremely small thickness. It has been shown that the absorption in the visible range under 1000 nm is definitely noticeable in spite of the small thickness and is disadvantageous especially for high quality electronic display apparatus, such as computer screens, particularly for laptops or also for mobile telephones.
Furthermore the transmission of or transparency to radiation, especially visible light, down to wavelengths below 400 nm, especially below 380 nm, should be kept comparatively constant and then rapidly reduced. Gas discharge lamps, especially fluorescent lamps, emit a strong fraction of their radiation in the UV range, which has a damaging effect on surrounding components, such as polymers and other plastic materials, so that they become brittle with increasing time, which can result in the failure of the entire product. The mercury line at 313 nm is an especially damaging emission line. Thus one goal is to make glass of this type, which absorbs this emission line as completely as possible.
Fluorescent lamp glasses for the above-described application, which absorb UV radiation to the desired extent, are known from U.S. Pat. No. 5,747,399. However it has been shown that this sort of glass is characterized by a strong discoloration in the visible range and strong solarization. Frequently a yellow brown discoloration is produced already during melting of the raw materials.
A zirconium oxide-containing and lithium oxide-containing borosilicate glass of high resistance is known from DE-A 198 42 942, which is especially suitable for use as solder or fusing glass with Fe—Co—Ni alloys.
One such glass can also contain color-imparting ingredients, such as Fe2O3, Cr2O3, CoO and TiO2.
U.S. Pat. No. 4,565,791 describes a glass for ophthalmologic applications, which has special refractive indices and Abbé numbers and densities suitable for that purpose. A glass of this sort has a UV-absorption limit between 310 and 335 nm and contains TiO2 as UV absorber. It is explicitly stated that refining with chloride is necessary in many cases in order to make this glass, since As2O3 and Sb2O3 refining is insufficient. Finally the reference likewise states that although the glass of this type is extremely thin a combination of Fe2o3 and TiO2 leads to discoloration of the glass, which is the reason that quartz material with an iron content of less than 100 ppm should be exclusively used.
Also borosilicate glasses with a small amount of B2O3 are known. For example, this sort of zirconium oxide-containing and lithium oxide-containing borosilicate is described in DE-A 198 42 942. This glass has a high acid and alkali resistance and resistance to hydrolysis and is especially suitable for sealing or soldering with Fe—Co—Ni alloys. This sort of glass can contain color-imparting ingredients, such as Fe2O3, Ce2O3, Co and TiO. However it is known that the boron content in this sort of glass leads to reduced resistance to chemical attack. Up to now glass with a high boron content, i.e. of more than 25% by weight, has not been considered for use as glass for gas discharge tubes, because it has an extremely poor chemical resistance, and, based on that, the fluorescent layer in these lamps would react with the substrate glass under the reactive conditions existing in the glass discharge tubes.